Ordinarily in thrillers dealing with multiple personality disorder, the condition itself serves as a late-film plot twist. Think (spoiler alert) Psycho, Fight Club, Shutter Island. Writer-director M. Night Shyamalan (The Sixth Sense, Unbreakable, Signs) is either the patron saint or the worst enabler of plot twists, depending on your opinion. But in his unnerving new film Split, he gets that information out right up front.

“When I was pitching the movie three years ago,” Shyamalan tells EW, “I was always teasing people with this scene: Three girls are trapped in a room, hearing a man and a woman arguing behind a door, and then the door opens and it’s just a man standing there. That’s how I would sell the movie.”

Split takes that as its jumping-off point, but then what’s next? Plenty, according to Shyamalan and the daring Scottish actor James McAvoy (Atonement, Filth, the last three X-Men films). In the exclusive video, above, McAvoy explains why his director couldn’t give him a straight answer when he asked for the name of his character. When McAvoy first appears he’s credited as Kevin — but that’s merely one of 23 people (at least) who inhabit the man’s brain.

Universal Pictures

Shyamalan, who’s been fascinated by Dissociative Identity Disorder since James Cameron announced (but later abandoned) a film on the subject in the early 1990s, also relates to how we all tend to compartmentalize. And to a certain extent, the voices in his own head.

“I am a writer,” he tells EW. “But then I walk on set as the director. Then I’m a producer. And occasionally an actor. Those hats swap like different personalities. I have to be careful of the others. The writer is cognizant that the director would like to have cinematic moments, perhaps at the expense of dialogue. The director looks at the script and goes, ‘Ugh, why did that writer write it like this?’”

Splitopens on Jan. 20. Check out the trailer (below) and stay tuned here for more news on the film.